Artificial acoustic reverberation with feedback and means for preventing sound discontinuittes



Oct. 27, 1964 w TAK 3,154,644

ARTIFICIAL ACOUSTIC REVERBERATION WITH FEEDBACK AND MEANS FOR PREVENTINGSOUND DISCONTENUITIES Filed NOV. 16, 1960 OSCILLATOR RECORD HEAD SOUNDSOURCE .5 5 PLAYBACK HEAD M j-E Fig.l

VARIABLE BIAS INVEN TOR. WILLEM TAK ,q AGENT United States Patent3,154,644 ARTIFICIAL ACOUSTIC REVERBERATION WITH FEEDBACK AND MEANS FORPREVENTING SOUND DISCONTINUITIES Willem Tak, Eindhoven, Netherlands,assignor to North American Philips Company, Inc, New York, N.Y., acorporation of Delaware Filed Nov. 16, 1960, Ser. No. 69,623 Claimspriority, application Netherlands Nov. 18, 1959 2 Claims. (Cl.179-100.1)

This invention relates to devices for producing artificial acousticreverberation by means of a recording head positioned along the path ofa moving sound track, followed by a plurality of reproducing heads eachof which controls one or a plurality of loudspeakers through anamplifier, with the signal of at least one reproducing head being fedback to the recording head.

Devices of this kind are known and have been manufactured and used bythe applicant and others. They have the disadvantage that adiscontinuity is distinctly audible in the reverberation produced. Sucha discontinuity is heard as an unnatural sound and is due to the factthat the ratio between the output energies of the reproducing headsremain the same for each cycle, resulting in a periodic repetition inthe reverberation.

This disadvantage could be obviated by proportioning the saidenergy-ratio and the degree of feedback so that the intensity of thesound produced decreases perfectly regularly. However, it is difiicultto realize this effect in practice. I

An object of the invention is to improve the described device producingreverberation so that the disadvantages relating to discontinuities areavoided.

The improvement according to the invention consists in that theamplification factors used for the feedback are made to vary differentlyrelatively to each other.

According to the invention, in a preferred embodiment thereof at leastone tube of each feedback amplifier has a mutual conductance which ishighly sensitive to small variations in grid voltage, and this mutualconductance is varied by applying to an appropriate grid of the tube asubaudible alternating voltage having a frequency which is different foreach tube.

If the invention is applied to a reverberation equipent in which, in themanner already carried out in practice, four reproducing heads arearranged around a wheel provided with a magnetic tape, it is possible,for example, to replace the first tube 'of the feedback amplifierassociated with the said heads by a tube of the type EF 85, a so-calledvariable-mu tube. The grid of this tube may then have applied to it asinusoidal bias protential having a frequency located below the audiblerange, for example of 5 c./sec.

One may proceed similarly for the feedback amplifiers associated withthe second, the third and the fourth head while taking care that thefrequencies of the bias potentials of these tubes, although likewiselying below the audible range, differ both from one another and from thefrequency used for the first amplifier. These frequencies may be, forexample, 7 c./sec., 9 c./sec. and 11 c./sec., respectively.

The invention will be better appreciated from the following descriptionwith reference to the drawing, wherein:

FIG. 1 is a schematic diagram of one embodiment of a reverberationsystem embodying the invention; and

FIG. 2 is a schematic diagram of one embodiment of the amplifier whichcan be utilized.

As shown in FIG. 1, the reverberation device includes and endless tape10 composed of magnetic material; this tape, of course, could be a drumor any other rotating medium having magnetic material on or attached toits surface. Coacting with the tape is a recording head 1, to whichsignals from the sound source are applied as shown, and reproducingheads 2, 3, 4 and 5. Each reproducing head supplies signals to aloudspeaker or a group of loudspeakers after amplification, as shown.For the sake of simplicity, only loudspeaker 12 and amplifier 11, showncoupled to head 5, are labeled in FIG. 1. In addition, to achieve thereverberation effect, the output from each reproducing head 2, 3, 4 and5 is fed back to recording head 1 through respective amplifiers 9, 8, 7and 6. According to the invention, to prevent the discontinuitiesmentioned above, oscillators 16, 15, 14 and 13 may be provided, each oneproducing a subaudible alternating signal and each one acting to changethe amplification factor of the respective amplifiers 9, 8, 7 or 6; theoscillators produce signals at different frequencies. The differentoscillators shown in FIG. 1 are representative only and any manner ofachieving different subaudible frequencies may be used, so long as theseare applied as shown to vary the amplification factor of the amplifiers.

FIG. 2 shows one embodiment of a feedback amplifier; a variable-mupentode 12 is shown, to the control grid of which are applied an input,which is the output of a reproducing head, and a source of variablebias.

Since the amplification factors of the feedback amplifiers vary withtime, but not simultaneously, the troublesome periodic repetition of aconstant energy ratio of the reproducing heads as was heard with theprior device is no longer present. Periodic repetition is no longeraudible in the reverberation produced by the device according to theinvention, this reverberation thus being considerably more natural.

In addition, although the energy ratio between the outputs of the fourheads is still bound within certain limits, it is less critical than inthe known device in which it did not vary with time.

Control of the reverberation time by varying the value of the meanfeedback factor remains possible within wide limits.

It is to be noted that the variable bias potentials applied to the gridsof the variable-mu tubes need not necessarily be sinusoidal, but may bereplaced by voltages varying according to other functions, provided thatsuch voltages do not contain audible components. It should also be notedthat various modifications and variations are possible without departingfrom the inventive concept, the scope of which is set forth in theappended claims.

What is claimed is:

1. A device for producing artificial acoustic reverberation comprising:a source of sound signals, recording means for recording said signals, aplurality of reproducing means for reproducing the recorded signals,part of the reproduced signals being fed back to said recording meansthrough a plurality of feedback amplifiers, and cyclical bias means forvarying the amplification factors of said feedback amplifiers, said biasmeans cyclically changing the amplification factor of each amplifier,the frequency of the cyclical change being different for each amplifier.

2. A device for producing artificial acoustic reverberation comprising:a source of sound signals, recording means for recording said signals, aplurality of reproducing means for reproducing the recorded signals,part of the reproduced signals being fed back to said recording meansthrough a plurality of feedback amplifiers each comprising an electrontube having a control electrode and a mutual conductance which is highlysensitive to small variations in the voltage on said control electrode,and a plurality of oscillators generating oscillations of subaudiblefrequency, each oscillator being coupled to the control electrode of afeedback amplifier, the frequency of said oscillations being diiferentfor each oscillator.

UNITED STATES PATENTS Daniel Dec. 19, 1950 Ambrose Apr. 6, 1954 LalfertyOct. 16, 1956 Vermeulen et a1. Apr. 5, 1960 FOREIGN PATENTS France June16, 1954

1. A DEVICE FOR PRODUCING ARTIFICIAL ACOUSTIC REVERBERATION COMPRISING:A SOURCE OF SOUND SIGNALS, RECORDING MEANS FOR RECORDING SAID SIGNALS, APLURALITY OF REPRODUCING MEANS FOR REPRODUCING THE RECORDED SIGNALS,PART OF THE REPRODUCED SIGNALS BEING FED BACK TO SAID RECORDING MEANSTHROUGH A PLURALITY OF FEEDBACK AMPLIFIERS, AND